The correct option is C utopia
“Utopia” means an imagined place or state of things in which everything is perfect, which matches with the statement given, hence option C is the correct answer.
Option A is incorrect because “idealism” means the unrealistic belief in or pursuit of perfection, so it makes sense to say that you can idealise utopia, hence it is incorrect.
Option B is incorrect because “cornucopia” means an abundant supply of good things of a specified kind, hence it is incorrect.
Option D is incorrect because “plethora” means a large or excessive amount of something, which is not the same as imagining a perfect society, hence it is incorrect.